The present invention relates to ultrasonic quantitative measurements and, in particular, to an improved apparatus and method for making ultrasonic measurements in axial material properties when only lateral access to the material is possible.
Conventional ultrasonic imaging provides a mapping of ultrasonic echo signals onto an image plane where the intensity of the echo, caused principally by relatively small differences in material properties between adjacent material types, is mapped to brightness of pixels on the image plane. While such images serve to distinguish rough structure within the body, they provide limited insight into the physical properties of the imaged materials.
Ultrasound “elastography” may be used to produce data and images revealing material properties such as strain under an externally applied stress, Poisson's ratio, Young's modulus, and other common strain and strain-related measurements. Quasi-static elastography provides these measurements by taking two images of a material in two different states of compression, for example no compression and a given positive compression. The material may be compressed by a probe (including the transducer itself) or, for biological materials, by muscular action or movement of adjacent organs. Strain may be deduced from these two images by computing gradients of the relative shift of the material in the two images along the compression axis. Quasi-static elastography is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the material and detecting the amount of material yield (strain) under this pressure.
Ultrasound “acoustoelasticity” provides an alternative method of computing material properties using a mathematical description of the relationships between sound propagation and material properties to deduce the properties more directly. For example, a measurement of reflected energy as ultrasound passes through a front and rear boundary of a material in tension can be used to deduce the strain of the material along the axis of tension and the stiffness of the material along the direction of ultrasound propagation. This technique, pioneered by the present inventors, is described in U.S. patent application 2007/0089530 filed Oct. 16, 2006 and hereby incorporated by reference.
The measurement of loads (stress) and stiffness in axial direction can be important in medicine, for example, in the assessment of disease or repair of ligaments and tendons. While the above described acoustoelastic technique allows the determination of axial strain (e.g. stretching along the length of the tendon) it does not measure the axial stress (e.g. tension along the length of the tendon) or axial material properties such as stiffness which may, particularly in tendons and ligaments, vary substantially between the axial and transverse directions.
While it is generally possible to measure axial properties simply by shifting the direction of the transducers to be aligned along the axis of measurement, this is not practical with in situ tendon and ligaments. Many important applications where measurement of axial properties of tendons and ligaments is desired, occur when the tendons and ligaments are under tension in the body and only lateral access to the tendon or ligament is available, for example, through thin overlying tissue or a surgical incision.